The present invention relates to a system for wireless, bi-directional transfer of electric signals between a host unit such as a data reader and a mobile guest unit such as an information carrier, particularly a packaging such as a cardboard box. While the electric signals primarily are used to represent digital data information, the system can also be used to transfer electric energy from the stationary unit to the mobile guest unit.
Prior art systems of this type are known from U.S. Pat. Nos. 4,876,535 and 5,847,447.
Today""s advances in mobile computing have created a vast amount of small and portable devices, generally operated on battery power. In applications ranging from cellular phones, handheld computers to data collection devices, such as electronic metering instruments, data loggers etc. sufficient data processing capabilities are now incorporated to perform direct data exchange between the portable unit and a host computer. Information is often transferred both ways, where parameter setup, transferring of memos and other field data is information provided from the host computer and results, such as measured values, are transferred back to the host device.
There are also passive portable devices, such as identity cards (Smart- or IC-cards) or packaging identifier, where the device is generally powered down when being out in the field and no battery power is provided. When attached to the host computer, the device is powered and can perform data exchange. Information is generally kept in non-volatile memory.
The straightforward way of performing data exchange is by direct cable connection, where several standardized interfaces and protocols exists. The CCITT V.24/V.28/EIA RS-232 is by far the most common electrical specification where several standardized and proprietary protocol mechanisms exist in controlling the data transfer process.
There is a general understanding that cable connections suffer several drawbacks, where the most obvious can be summarized as;
A slow manual process of aligning, attaching and detaching cable connectors.
Mechanical degradation of contacts and contact elements.
Environmental degradation due to humidity, water, dirt and corrosion.
Open slots, which expose vital parts of the device for dirt and electromagnetic interference, such as electrostatic discharge.
In many applications it is desirable to perform data transfer in a wireless manner. Several commonly used methods are available, such as Radio Frequency (RF) and Infrared radiation. RF devices have the obvious benefit of being able to transmit information over long distances, but generally suffer from high power requirements and careful selection of antenna and oscillator design to maintain selectivity and not interfering with other devices in the public bands. Infrared beams have the benefit of being simple to implement, but requires careful alignment and clear sight between the transmitting and receiving ends.
Where a close-proximity relationship exists between two devices, there are several proprietary methods developed for transmitting information, relying on the near-field effects of electromagnetic wave propagation. Generally, this is divided into inductive (magnetic field) propagation and capacitive (electric field) propagation. Some methods include a combination of both.
Magnetic field propagation relies on energizing a first coil with alternating current, where, magnetic energy is radiated. By placing a second coil in proximity to the first coil, an inducted current generates an alternating voltage over the second coil.
Capacitive field propagation relies on applying an alternating voltage on a first electrical conductive surface. By approaching a second electrical conductive surface to the first, the electrostatic charge between the surfaces in the form of an alternating voltage can be measured between the second surface and a common ground. To obtain a current flow to a portable device, a corresponding second set of conductive surfaces needs to be formed to close the loop.
As the impedance of an inductance increases with frequency, good magnetic coupling is achieved at lower frequencies. The drawback with inductive transfer in portable systems is the high-energy losses, which primary relates to resistive and flux losses in a coil. Also, the manufacturing of coils is relatively expensive.
In contrast, the capacitor impedance decreases with increasing frequency. The loss in a practical capacitor is low in comparison to a coil, due to low values of serial and parallel resistances. The drawback with capacitive transfer is the need for high voltages and large surface areas in order to achieve good coupling, as the capacitance decreases as the distance between the capacitive surfaces increase.
Prevailing systems for capacitive data transfer rely on good capacitive coupling between the devices. In applications where the electrically conductive material used to form the capacitive element is poor in terms of resistive conductivity or the distance and/or dielectric properties of the medium between the capacitive elements, where air is considered equal to additional distance, these methods are not sufficient for proper operation.
In some applications addressed by the present invention the following characteristics are desired:
Micropower quiescent current requirements. The portable device should preferably have virtually zero quiescent current.
The host device interface should be low power in order to be able to be powered from the small amount available from a V.24/V.28 serial port.
Must work properly on distances up to a few millimeters.
Must work properly even if the coupling surface is not perfectly flat.
Must work properly independent of the dielectric medium present between the devices.
Must work properly where the capacitor plates are made of poor electrically conductive material, such as conductive polymers, graphite, or Indium-Tin Oxide (ITO).
Should be relatively insensitive for misalignments of transceivers.
Should preferably be insensitive for rotational displacements in steps of 180xc2x0 .
Be simple and inexpensive to implement and not require any manual tuning or rely on narrow tolerance component values.
An object of the invention is to provide a low-cost system for wireless, bi-directional transfer of electric signals over a capacitive interface which allows for a high impedance in the circuitry of the guest unit in order to obtain a good signal transfer ability in conditions of poor dieelectric materials, poor conductivity in the contact pads and relatively large gaps between the contact pads.
Another object is to provide a system which allows for the mobile unit to be rotated 180 degrees so that cooperating pairs of the contact pads may be unintentionally shifted without loss of functionality. This is of importance when transferring information between box-shaped packages and a stationary unit.
According to an aspect of the invention there is provided a system for wireless, bidirectional transfer of electric signals over a capacitive interface formed between electric circuitry contained partly in a host unit and partly in a guest unit when the units are placed in a proximity relationship. The capacitive interface comprises a respective first, second and third conductive area in the host and guest units. The first conductive area of the host unit is connected to a frequency generating resonant circuit in the host unit for coupling high amplitude signals transmitted to the guest unit. The second and third conductive areas of the host unit are connected to an impedance circuit in the host unit for receiving signals from the guest unit. The first and second conductive areas of the guest unit being connected to an impedance circuit in the guest unit for receiving signals from the host unit. Also, the first and third conductive areas of the guest unit are interconnected.
The frequency generating resonant circuit provides a carrier output to the first conducting area of the host unit. By the resistive feedback this design provides for an automatic tuning of the resonant circuit to operate at its peak output amplitude, relatively independent of the complex impedance loading of the conductive area. By this arrangement there is obtained a relatively high amplitude output from the host unit. The circuitry and conducting areas, particularly in the host unit can therefore be fabricated from non-expensive relatively low-conductive materials, such as conducting polymer materials, which can be applied by printing to the substrates for the circuitry and the conducting areas.
By interconnecting the first and third conductive areas of the guest unit, a side of the capacitive interface is allowed to be rotated in half-turns without loss of signal transfer function when the conductive areas are arranged consecutively in a line.